The Electronic Frontier Foundation has asked the U.S. Supreme Court to intervene in its effort to obtain a copy of a secret government memo authorizing the FBI to collect phone records.

The EFF, a digital rights group, on Tuesday asked the Supreme Court to decide whether a January 2010 U.S. Department of Justice Office of Legal Counsel [OLC] opinion, apparently allowing the FBI to informally collect phone records from telecom carriers without further legal authority, should be made public.

The OLC opinion "establishes the scope of the executive branch's authority to obtain private communications records without legal process or a qualifying emergency, despite apparent statutory prohibitions to the contrary," EFF's lawyers wrote in their request to the Supreme Court.

The FBI's informal telephone records requests, revealed in a 2010 report by the DOJ's Office of Inspector General, is separate from the U.S. National Security Agency's controversial telephone records collection program revealed in leaks by former NSA contractor Edward Snowden.

The OLC opinion appears to rely on legal authority that is different than that used by the NSA program, which cites the Patriot Act, said Mark Rumold, an EFF staff attorney."In fact, we can't say with 100 percent certainty what statute the [FBI] collection authority is based on, because they've kept that secret," he said by email.

A DOJ spokesman declined to comment on the EFF's Supreme Court request.

In addition to the FBI, the CIA may be using the OLC memo to collect telephone records, based on news reports from late 2013, Rumold said.

In May 2011, the EFF filed a Freedom of Information Act lawsuit against the DOJ to obtain the memo, but the agency has resisted the request. The DOJ has prevailed in district court and on appeal.

EFF's lawyers argue the public should have access to important OLC memos.

"The public has an unquestionable interest in access to OLC's interpretations of law," they wrote in their Supreme Court request. "OLC opinions often represent the final word on the legality of executive branch action and provide powerful shields of legal immunity to executive officials relying in good faith on their conclusions."

The DOJ has interpreted court rulings in the case "to authorize a near-categorical shield of privilege for these important documents," they added.

Grant Gross covers technology and telecom policy in the U.S. government for The IDG News Service. Follow Grant on Twitter at GrantGross. Grant's email address is grant_gross@idg.com.